1. Field of the Invention
This invention generally relates to nuclear meters, and more particularly to improvements in neutron-capture-based elemental analyzers leading to an increased uniformity of response in on-line analytical measurements of bulk substances.
2. Description of the Prior Art
The rising cost of fuels, coupled with the need to avoid atmospheric pollution when burning them, has led to the requirement that their composition be known at various points in the fuel-preparation cycle. For example, because of the scarcity of low-sulfur crude oils and the cost of sulfur removal, the value of fuel oil increases significantly as its sulfur content becomes lower, indicating that accurate fuel-oil blending to a fixed sulfur level consistent with allowable amounts of pollution is both cost effective and an efficient utilization of increasingly-scarce hydrocarbons. Furthermore, precise knowledge of the heat content of fuel oil allows furnaces and boilers to be operated in a more-efficient manner. In addition, knowledge of the amount of sulfur and other contaminants such as vanadium and nickel in various hydrocarbon streams can help prevent the poisoning of catalysts used in oil refineries, avoiding costly shut downs.
In the case of coal, sulfur content is generally higher than that of oil, making the pollution problem even more severe. As a result, expensive coal-cleaning plants, stack-gas scrubbers and precipitators are necessary, all of which can be operated more efficiently if the coal composition is known on a real-time, on-line basis. Efficient boiler operation also benefits from this composition measurement, and knowing the composition of the ash in the coal can be used to avoid boiler slagging, which is a costly problem that is generally absent for fuel oil.
Particularly in the case of coal, but also for oil, these composition measurements have to be made on inhomogeneous substances with high mass flow rates and variable compositions. Thus, this measurement should continuously reflect the average composition of the bulk substance, and response times should be fast enough to permit effective process control. Generally the latter requirement implies a speed of response ranging from a few minutes up to an hour.
A technique which can satisfy these requirements can often be used in applications which do not involve fuels or their derivatives. For example, it could measure the nitrogen content of wheat in order to determine the amount of protein present, which in turn is related to food value. Thus, the measurement of fuels is illustrative only and is not essential to this invention, which applies to all measurements of bulk substances by the techniques to be described hereinafter.
Several methods for composition measurement are known in the prior art, the most obvious one being sampling following by chemical analysis. This technique provides most present data on the composition of various bulk substances. Unfortunately sampling is inherently inaccurate because of the lack of homogeneity of bulk materials, and large continual expenditures for manpower, sampling devices and chemical-analysis equipment are required to provide response times which at best could approach one hour. These disadvantages lead to the consideration of other techniques which are faster, more subject to automatic operations and more of an on-line continuous bulk measurement.
One technique often used in industrial environments for elemental analysis involves X-ray fluoresence. This technique relies on the fact that each atom emits X-rays with distinct and well-known energies when external radiations disturb its orbital electrons. Unfortunately, sulfur, which is an interesting element from the standpoints of air pollution and catalyst poisoning, emits mostly 2-keV X-rays, which can only traverse about 0.1 mm of a typical fuel. Iron, which is one of the elements generating the highest-energy X-rays in coal, produces mostly a 6-keV X-ray, which also cannot escape from any appreciable amount of coal or other nongaseous fuel. Thus, the use of X-ray fluorescence for other than gaseous materials requires either the preparation of very clean surfaces truly representative of the bulk material or the vaporization of a sample in an atmosphere which does not confuse the measurement. In either case, a difficult sample-preparation problem compounds the errors associated with X-ray fluorescence itself.
A second technique usually involving X-rays which are more penetrating is X-ray absorption. In this case one measures the differences in the absorption or scattering of X-rays caused by changes in the amounts of certain elements. In the case of relatively-pure hydrocarbons such as refined fuel oil, this technique can provide a useful measurement of sulfur content because sulfur at X-ray energies near 22 keV can have a predominant effect on the X-ray absorption. This predominance, however, is dependent on the lack of most of the metals which are present in coal and may also be present in oil. In addition, 22-keV X-rays only penetrate about 2 mm in most non-gaseous fuels, making sampling still a requirement. Moreover, this technique is generally limited to measuring only one of several potentially-interesting elements, and the measurement of the relative amount of many different elements in a complex mixture such as coal becomes difficult.
Nonetheless, nuclear techniques in general remain attractive because they often can be automated and in principal do not require actual manipulation of the bulk material itself. The problems with X-ray fluorescence and absorption arise partly because the associated radiations are not sufficiently penetrating. However, because the energetic gamma rays produced by the capture of thermal neutrons will penetrate over 100 mm of most fuels, an analysis technique based on them can provide an accurate, continuous, on-line measurement of the elemental composition of bulk substances without sampling.
This technique is based on the fact that almost all elements when bombarded by slow neutrons capture these neutrons at least momentarily and form a compound nucleus in an excited state. Usually the prompt emission of one or more gamma rays with energies and intensities which are uniquely characteristic of the capturing nucleus dissipates most of this excitation energy. Because these prompt gamma rays often have energies in the 2- to 11-MeV range, they can penetrate substantial quantities of material. Thus, for those isotopes with significant capture cross sections and prominent gamma-ray lines, measurement of prompt gamma rays can be used to determine in an on-line, real-time manner the quantity of most of the elements present in bulk substances, which can be flowing through the analyzer.
The above emphasis on thermal neutrons reflects the fact that for most elements the cross section for neutron capture is approxiately proportional to the reciprocal of the square root of the neutron energy. Thus, almost all neutron capture occurs at the lowest neutron energies, which happen when the neutrons are in thermal equilibrium with the nuclei of the surrouding medium. As a result, the thermal-neutron-capture cross sections characterize the expected prompt-gamma-ray spectra. These gamma-ray spectra are particularly amenable to simple theoretical interpretation using well-known thermal-neutron-capture cross sections, making automatic operation a feasible concept.
However, because isotopic and other neutron sources generally produce neutrons with average energies of at least several MeV, "moderation" or "thermalization" processes must reduce neutron energies by over eight orders of magnitude in order for them to reach the thermal region near 0.025 eV. Collisions with hydrogen nuclei, which have a mass essentially the same as that of the neutron and a large scattering cross section, are the most effective means for neutron moderation, although collisions with other elements will moderate neutrons to some degree. Because the neutrons move between collisions, the volume of material exposed to significant neutron fluxes can have a considerable extent, which depends mostly on the amount of hydrogen present. Because the thermal neutrons are produced continuously by moderation of the more-energetic neutrons and then diffuse throughout this moderation volume, the substance being measured is sampled over a large extent, providing the bulk measurement.
Although these techniques have been used in the laboratory under controlled conditions, their implementation in an automatic, on-line instrument placed in an industrial environment presents unique problems which prior-art instruments have not solved. One of these problems arises when the instrument must measure accurately substances with compositions which vary within the measurement volume. One such non-uniform material may occur when coal with various particle sizes of different compositions flows through a chute or channel passing through the instrument. Often in this case the coal particles will differentially segregate along the chute walls so that the elemental composition depends upon position within the measurement volume. Only if the instrument has a response which is independent of position within the measurement volume will it measure correctly the average composition of the bulk substance. Incomplete mixing of fluids or slurries could produce the same problems as illustrated above for solid coal, again leading to the need for a uniform response or measurement sensitivity.
In the known prior art, a single neutron source was located in the center of a coal chute passing through the measurement volume, and a single gamma-ray detector, which was located outside of the chute, was used to measure the energy spectrum of the capture gamma rays. In the plane passing through the center of the source and detector, the measurement sensitivity in this configuration varied both along the source-detector line and perpendicular thereto. Three major effects led to these sensitivity variations.
First, the neutron flux decreased as the distance from the source increased, and this flux had to fall substantially at the sides of the chute compared to its center in order to have an acceptable number of neutrons escaping into the detector. Because the probability of producing a gamma ray is proportional to the flux of thermal neutrons, the production probability per unit volume therefor also had to be substantially less at the sides of the chute than at its center.
Second, the solid angle subtended by the detector for each small volume in the coal chute is less for those volumes distant from the detector than for those closer to the detector. As a result, the probability that a gamma ray emitted from such a volume reached the detector decreased for volumes near the source or the far wall of the chute, and for those volumes removed from the source-detector line, compared to volumes on the source-detector line and near the detector side of the chute.
Third, the probability that a gamma ray moving toward the detector can travel from the region where it was produced to the detector without interacting decreases as the distance which it must travel increases. Thus, the measurement sensitivity, which was determined primarily by the gamma rays which did not interact, was less for regions near the source of the far wall of the coal cute compared to regions near the detector.
In these prior-art instruments all of these effective combined to make the sensitivity at the side of the chute on the opposite side of the source as the detector considerably less than the sensitivity in the volume between the source and the detector. However, if the neutron source is located outside of the measurement volume as described in another application for a U.S. patent, Ser. No. 808,106, filed on June 21, 1977 by the inventor herein, then the measurement can be confined to the volume between the source and the detector, resulting in improved uniformity. However, unless the techniques of this invention are also employed, the uniformity of sensitivity may still be insufficient to measure accurately segregated, inhomogeneous substances.
For example, if only a single source and detector are used, both the neutron flux and the solid angle are lowest at the sides of the chute compared to its center along a direction perpendicular to the source-detector line. Furthermore the amount of scattering material through which the gamma rays produced at the sides of the chute must travel to reach the detector is greater, adding to reduced sensitivities at the chute sides compared to its center. This effect becomes more severe as the chute becomes larger, particularly if neutron-absorbing materials or open spaces surround the chute and further depress the thermal-neutron flux at the chute sides. Large chute sizes may result from high mass flow rates or from other geometrical constraints imposed by the industrial environment and various properties of the substance being analyzed.